1. Field of the Invention
The present invention relates to a method of and an apparatus for controlling deflection of an optical beam through an AOD (acousto-optic deflector), and more particularly, it relates to a deflection control for constantly maintaining diffraction efficiency of an AOD at any deflection angle.
2. Description of Prior Arts
An AOD is generally formed by an ultrasonic oscillator and an acoustic element, and an optical beam is incident upon the acoustic element. When the ultrasonic oscillator oscillates an ultrasonic wave toward the acoustic element, diffraction takes place in the acoustic element to output primary diffracted light. The outgoing angle (i.e., deflection angle) of the primary diffracted light depends on the frequency of the ultrasonic wave. Thus, the deflection angle of the outgoing light is varied with frequency change of the ultrasonic wave with time.
It is known that diffraction efficiency of such an AOD depends on the deflection angle. In other words, the intensity of outgoing light from the AOD is varied with the frequency of an ultrasonic wave deciding its deflection angle assuming that the intensity of incident light upon the AOD is constant. Therefore, the intensity of the outgoing light from the AOD is inevitably varied with the deflection angle if an ultrasonic wave having a constant amplitude is oscillated from the ultrasonic oscillator and the frequency thereof is changed. This causes a problem in scanning for exposing of a photosensitive material, such as a photosensitive film, since recorded density is varied with the scanning positions and constant image density cannot be obtained.
Japanese Patent Laying-Open Gazette No. 59-160128 discloses a technique for solving this problem. This technique is based on the fact that intensity of outgoing light from an AOD depends not only on the frequency, but also the amplitude of the ultrasonic wave, and corrects the intensity of the outgoing light, where the word "intensity of outgoing light" is used in the present specification under the condition that the intensity of light incident to an AOD is constant. The amplitude of the ultrasonic wave is proportionate to the power or amplitude of an input signal which is supplied to the ultrasonic oscillator. Thus, the power of the input signal is changed in response to the frequency of the ultrasonic wave (i.e., the frequency of the input signal), in order to attain the highest diffraction efficiency at each frequency value.
In this technique, a plurality of power levels are first set for the input signal to the ultrasonic oscillator, and the frequencies of the ultrasonic waves are changed at the respective power levels, thereby obtaining the relation between the frequency and diffraction efficiency. Then, relations between the frequencies obtained for the respective power levels and values of diffraction efficiency are compared with each other, to obtain power levels maximizing the values of diffraction efficiency at the respective frequencies. Thus, considerable time and labor is required since the relation between the frequency and the diffraction efficiency is obtained at every power level.
Further, since diffraction efficiency is also varied with environmental conditions such as the temperature of the AOD, it is necessary to also change the power of the input signal to the ultrasonic oscillator in order to constantly maintain the intensity of outgoing light from the AOD if the environmental conditions are changed. In the aforementioned technique, however, it has been difficult to finely adjust the power of the input signal in response to change in the environmental conditions since the power of the input signal to the ultrasonic oscillator has been corrected by a filter circuit.